Yield losses in semiconductor integrated circuit manufacture occur at various stages in the manufacturing process. Those that occur during packaging, the final operation, are especially expensive because of the substantial investment already made in the device at that stage of its manufacture. A troublesome defect in plastic packaging are voids in the molded part. Not only are voids unacceptable from the standpoint of the integrity of the package, but the entrained gas that causes the void often produces a hydrostatic pressure shock sufficient to destroy one or more wire bonds between the chip and the lead frame.
In conventional molding of plastic packages the thermosetting resin is preheated prior to introduction into the molding apparatus. The resin is preheated in the for of large pellets or tablets. The preheated tablets are placed in a cylinder, or transfer pot, and a piston is used to produce the hydrostatic pressure necessary to transfer the molding resin through the distribution channels, or runners, to the individual mold cavities. As a result of substantial experience in observing this operation, and studying the formation of unacceptable voids, I reached the conclusion that voids formed in the plastic package occur as the result of air entrapped in the transfer resin while it is in the transfer pot. I have also concluded that air will unavoidably be trapped in the charge if the charge is preheated according to the standard practice in the art.